While I do dislike this move on Harper's part, I think that jimmoyer raises a good point on "what would we do in the leader's shoes?".
Keep in mind that, according to the reports that I read, Emerson went to Harper... not the other way around. I doubt that leaders rarely, if ever, go around trying to recruit the MPs from other parties but that would look terribly slimey should it get out in the public.
Now... you're Harper. You have a very slim minority where every seat helps. B.C. is the one province where your party actually lost seats in the last election. David Emerson is considerred to be one of the major reasons for why the Liberals did well there in the last election. Emerson is also one of the few Liberal cabinet members that you liked during the last Parliament. He's coming to you, offering to cross the floor for a cabinet position. What do you do? If you tell him yes, that might tick off some backbenchers (who otherwise may have been cabinet ministers), and some people like us... but you have one more seat, stolen a leadership candidate from the Liberals, and gained one of BCs most popular politicians as well - potentially helping you in the only province you actually lost seats in for the next election.
If you say no, you make people like us happy, but if the party brass catches wind of your refusal to take on an extra seat, they may get extremely upset with you, asking "What the heck are you thinking? Every seat we could add is instrumental!".
So, either way, there's pros and cons.
Now, all of that being said, I do strongly dislike this, because I'm against people crossing the floor. When you vote for a candidate in your riding, you're not voting for just a candidate - you're voting for the candidate, the party, and the party leader. Sometimes the candidate may be the most important factor, but often it's not.
Many here have speculated that Emerson wouldn't win if he was running as a Conservative. That's quite possibly true. If Peter Kent had ran for the Liberals in the GTA, he would have won in a landslide. If Anne McLellan was a life-long Tory, she would have owned her seat in Alberta. Come on - we all know it's true. Party affiliation is often much more important in voter's minds than the actual riding candidate.
I am disappointed in Harper here (moreso with the Fortier Cabinet appointment actually - I really hate that move, and can honestly say that I would NOT have done that if I was in Harper's shoes; I'm honestly not sure what I would have done if Emerson came asking me for a cabinet position in return for crossing the floor).
So... unlike Liberal supporters who refuse to admit wrong-doing over the Dingwall situation, I'm willing to admit when the party I voted for is wrong. Harper is wrong here - on principle with Emerson (though pragmatically, I see why he did what he did) - and both in principle, AND pragmatically, on Fortier (I may have more to say on that later).
However, on cleaning up government, having fewer Belinda Stronach's isn't my hightest hope - having fewer David Dingwall's IS. I still hold out hope that Harper can clean up that aspect of government.